Field of the Invention
The disclosure relates to the field of air conditioning, and in particular relates to a two-stage compression air conditioning system and a method of controlling gas replenishment thereof.
Description of Related Art
A lower ambient temperature results in a greater demand for air-conditioning heating capacity. However, current single-stage compression heat pumps can be normally started to operate only at 20 degrees below zero, the heating capacity is greatly reduced, the heating effect cannot be guaranteed, and the reliability of the air-conditioning unit is also subject to a severe test.
Compared with a single-stage compression heat pump, a two-stage compression air-conditioning system with enhanced vapor injection has a greater heating capacity and a higher energy efficiency at a low temperature. Moreover, the two-stage compression air conditioning system can reduce the pressure ratio of the single-stage compressor, lower the exhaust temperature, and also improve the intake efficiency and compression efficiency, thereby improving the heating capacity and heating efficiency.
A two-stage compressor with enhanced vapor injection is divided into a high-pressure stage and a low-pressure stage. The two-stage compressor has two or more cylinders, wherein the cylinder for the first-stage compression is called a low-pressure cylinder and the cylinder for the second-stage compression is called a high-pressure cylinder. The principle of enhanced vapor injection is to inject a gaseous refrigerant into the gas intake of the high-pressure cylinder of the compressor from the enhanced vapor injection port in the middle portion of the compressor. The injected gaseous refrigerant will be mixed with the refrigerant discharged after being compressed by the low-pressure cylinder and then enter the high-pressure cylinder for compression.
In a two-stage compression air-conditioning system, gas injection in the middle has a very important influence on the system performance and reliability. With the difference in the compression ratios of high and low pressure stages and the change of working conditions, back flow of replenished gas is likely to occur in the two-stage compression system. The back flow of replenished gas means that, when the gas replenishing valve is opened, since the intermediate pressure of the flash evaporator is lower than the exhaust pressure of first-stage compression, part of the exhaust of the first-stage compression is directly discharged into the intermediate flash evaporator. Backflow of replenished gas greatly affects the reliability and performance of the two-stage compression system. When the exhaust of the first-stage compression is directly discharged into the flash evaporator, the lubricating oil will be discharged together, which is likely to cause oil starvation and wear of the compressor. In terms of performance, after entering the flash evaporator without being condensed and throttled, the high-temperature and high-pressure exhaust is mixed with the low-temperature and low-pressure refrigerant that has been condensed and throttled, which increases the temperature and pressure of the refrigerant before entering the flash evaporator, which correspondingly reduces the quantity of outdoor heat exchange and directly leads to the reduction of the quantity of indoor heat exchange and the deterioration of the energy efficiency of the system.
Related two-stage compression air conditioning systems do not determine and control back flow of replenished gas, so it is difficult to avoid the occurrence of back flow of replenished gas.